1. Technical Field of the Invention
The present invention relates to voltage regulators, and more particularly to voltage regulators for microprocessors and other applications which impose stringent demands on power supply performance. A typical modern microprocessor requires an operating voltage below 2 volts, which is stable within a quite narrow window, and currents which can switch between 0 and 50 or more amperes at a very rapid rate, and state of the art power supplies are even required to provide currents of about 100 amperes at about 1 volt, with voltage accuracies of under 1.0 percent. Very sophisticated power supply design techniques must be employed to maintain stable voltage under such conditions.
A preferred technique for controlling the output of microprocessor power supplies is active voltage positioning (AVP), the basic concept of which is illustrated in FIG. 1. According to this, the power supply output voltage is controlled as a function of the load current within an acceptable voltage window 8 defined by maximum and minimum voltages Vmax and Vmin, respectively. If the output current required is low, as at 10, the output voltage is adjusted to near the top of the voltage window, so the voltage does not drop below Vmin when there is a step output current increase. Conversely, if the output current is high, as at 12, the voltage is adjusted to near the bottom of the voltage window, so the voltage does not rise above Vmax when the output current returns to a low level.
2. Relevant Art
Switching regulators employing pulse width modulation (PWM) are generally used as microprocessor power supplies. When AVP is employed, an integrated circuit (IC) controller with built-in AVP circuitry is provided. The main features of such a controller is illustrated in FIG. 2, generally denoted at 18. These include an error amplifier 20, a compensation network 22, a PWM generator 24, and at least one pair of high and low side transistor switches such as MOSFETS 26 and 28, which provide an output voltage Vo to a load through an inductor-capacitor circuit 30. The load is represented by a microprocessor CPU 32, but those skilled in the art will understand that this is representative of any high current low voltage load characterized by rapid current transients which require a very stable voltage from the power supply.
For control, a signal representing voltage Vo is fed back to error amplifier 20 through an input resistor 34. As will be understood skilled in the art, this signal is normally provided by a current sense element (not shown) such as shunt resistors or the drain to source resistence of the fully conductive MOSFET, Rdson, or in any other suitable manner.
A current source 36 within IC 18, which operates in response to the current feedback signal, injects a current signal into the input resistor 34 to create a voltage offset and produce AVP window 8 (See FIG. 1).
This conventional approach has two major disadvantages. First, creating an ideal AVP output voltage waveform as illustrated in FIG. 1 requires optimal settings of the gain and loop response of both the error circuit and the AVP circuit. and AVP loop response. The illustrated method does not allow the user to easily optimize both loop responses.
Secondly, input resistor 34 of error amplifier 20 is also part of the compensation network 22 which stabilizes the control loop under all operating conditions to prevent oscillation. Since the value of resistor 34 is normally set at a fixed value to establish the AVP window, this forces the user to select all of the other resistors and capacitors in compensation network 22 based on the value chosen for input resistor 34. Changing the value of resistor 34 to accommodate design changes in compensation network 22 causes the AVP window to change, thus making compensating the system cumbersome.
Accordingly, there is a need for a more flexible solution for achieving AVP in power supplies for microprocessors and the like.